1. Field of the Invention
The present invention relates to a zoom lens preferably usable in optical apparatuses, such as a digital still camera, and a video camera.
2. Related Background Art
In recent digital still cameras, home video cameras, and the like, size and weight have been reduced. In conformity therewith, the size of photo-taking zoom lens has been remarkably decreased. Especially, efforts have been made to shorten the entire length of the lens, reduce the size of its front lens, and simplify its structure.
As a method for achieving those purposes, there has been known a so-called rear focus type zoom lens in which focussing is performed by moving lens units other than a first lens unit on the object side.
Generally, the effective diameter of the first lens in the rear focus zoom lens is smaller than that of a zoom lens which performs focussing by moving the first lens unit, and the size of the entire lens system of the former can be reduced more readily than that of the latter. Further, macro photo-taking, especially super macro photo-taking, can be more readily performed by the former. Additionally, in the former zoom lens, only a small driving force for driving a focussing lens unit is required for its actuator since focussing is performed by moving a small and light-weight lens unit, and focussing can be performed more rapidly than the latter zoom lens which needs to move a larger lens unit, provided that the same actuator is used.
For example, Japanese Patent Laid-Open Nos. 62-24213 and 63-247316 disclose rear focus type zoom lenses in which there are arranged, in the order from an object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, the power is varied by moving the second lens unit, and correction of variation of an image surface accompanying the variation of power, and focussing are performed by moving the fourth lens unit.
Further, Japanese Patent Laid-Open No. 8-5913 (U.S. Pat. Nos. 5,847,882 and 6,094,312 are its U.S. counterpart) discloses a rear focus type zoom lens in which there are arranged, in the order from an object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, a fourth lens unit having negative refractive power, and a fifth lens unit having positive refractive power, the power is varied by moving the second lens unit, and correction of the image surface variation accompanying the power variation, and focussing are performed by moving the fourth lens unit.
Further, for purposes of improving a containing capability at non-use (non-photo-taking) time of a camera, it is effective to retract the entire lens system into a camera body. However, in zoom lenses of the above-discussed zoom types in which the second lens unit performs most of power variation, it is unsuitable to retract the lens system since sensitivities of first and second lens units to decentering are too great.
In connection with the above, Japanese Patent Laid-Open No. 10-62687 (its U.S. counterpart is U.S. Pat. No. 6,016,228) discloses a zoom lens with a zoom ratio of about three, in which there are arranged, in the order from an object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, the first lens unit is composed of a single lens, zooming is performed by moving second, third and fourth lens units, and focussing is performed by moving the fourth lens unit. The entire optical system of the zoom lens is simplified, and suitably usable in a retractable structure.
In those zoom lenses, however, on-axis chromatic aberration and chromatic aberration of magnification are likely to increase specially at the telephoto end when the size of the entire lens system and the number of lenses are intended to be reduced. Therefore, conventional zoom lenses are difficult to employ in digital still cameras and video cameras using a pickup element with high-density pixels.
To cope with the above problem, Japanese Patent Laid-Open No. 2000-267005 proposes a zoom lens in which there are arranged, in the order from the object side, four units having positive, negative, positive and positive refractive powers, and a lens formed of an anomalous-dispersion glass material is used in the first lens unit.
On the other hand, the use of a diffractive optical element in a photo-taking optical system has been recently proposed as a method of suppressing chromatic aberrations.
In Japanese Patent Laid-Open Nos. 4-213421 (its U.S. counterpart is U.S. Pat. No. 5,044,706) and 6-324262 (its U.S. counterpart is U.S. Pat. No. 5,790,321), for example, a diffractive optical element is applied to a single lens to reduce the chromatic aberration.
Further, U.S. Pat. No. 5,268,790 proposes the use of a diffractive optical element in the second or third lens unit of a zoom lens such that the number of lenses and the size of the lens system can be made smaller than those of conventional zoom lenses.
Further, in Japanese Patent Laid-Open Nos. 11-52238 and 11-52244, for example, a diffractive optical surface is provided in a first lens unit to reduce the number of lenses in the first lens unit.
Further, in Japanese Patent Laid-Open No. 11-305126, a diffractive optical surface is provided on a jointed lens surface (cemented surface) between two lenses to reduce the chromatic aberration.
Further, when the diffractive optical element is employed in a photo-taking system, a sufficient diffraction efficiency needs to be attained over the overall visible range. In general, the diffraction efficiency of a single-layer diffraction grating decreases at wavelengths other than a designed wavelength, and unnecessary diffracted light in orders other than a designed order causes color flare. Considering such disadvantage, Japanese Patent Laid-Open No. 9-127322 (its U.S. counterpart is U.S. Pat. No. 6,157,488) proposes a system in which three different materials, and two different grating thicknesses of diffraction gratings constituting the diffractive optical element are selected in such an optimum manner that high diffraction efficiency can be obtained in the overall visible range.
Generally, in zoom lenses with high zoom ratio of about ten (10), when the diffractive optical surface is introduced into a first lens unit to correct the chromatic aberration, the angle of light rays incident on the diffractive optical surface greatly changes depending on variations of the angle of view and the focal length. Accordingly, the diffraction efficiency changes, and unnecessary diffracted light hence increases. In structures disclosed in Japanese Patent Laid-Open Nos. 11-52238 and 11-52244, since the diffractive optical surface is provided on a lens surface convex toward the image side, the angle of on-axis light rays incident on the diffractive optical surface largely differ from that of marginal rays. Hence, a sufficient diffraction efficiency cannot be obtained.
Furthermore, when a high diffraction efficiency is intended to achieve in the overall visible range, it is necessary to construct the diffractive optical element of a plurality of diffraction gratings, and set the diffractive optical surface on a cemented surface, on forward and rearward sides of which lenses are arranged. In Japanese Laid-Open No. 11-305126, a diffraction grating is provided on a cemented lens surface, but no incidence condition on the diffraction grating is considered. Therefore, the incidence condition of light rays on the diffraction grating largely changes depending on a change in the angle of view.
It is an object of the present invention to provide a novel zoom lens which can enjoy high optical performance over an overall zoom range.
In one aspect, a zoom lens system of the present invention includes a first lens unit having optical power, and a second lens unit having negative optical arranged on the rearward side of the first lens unit. In this zoom lens system, the first or second lens unit has a cemented surface (jointed surface) convex toward the forward side, and a diffractive optical portion is provided on the cemented surface.
In a preferred embodiment where the diffractive optical portion is provided on the cemented surface of the first lens unit, the condition of 0.5 less than RD/f1 less than 1.2 is satisfied where RD is the radius of curvature of the cemented surface, and f1 is the focal length of the first lens unit.
In another preferred embodiment where the diffractive optical portion is provided on the cemented surface of the second lens unit, the condition of xe2x88x922.8 less than RD/f2 less than xe2x88x921.2 is satisfied where RD is the radius of curvature of the cemented surface, and f2 is the focal length of the second lens unit.
These and further aspects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof in conjunction with the accompanying drawings.